On-going geo-political concerns, concerns for global warming, and rising oil prices are fueling the push for renewable energy sources such as wind and solar power. Today, the majority of the electricity generated in the United States is produced by burning fossil fuels, such as coal, natural gas, and petroleum, nuclear power and hydroelectric power. Energy produced from alternative energy sources, such as wind and solar power, account for a small percentage of the total electricity produced in the United States. Our reliance on fossil fuels and nuclear power has several drawbacks. While fossil fuels are comparatively inexpensive, there is only a limited supply of fossil fuels, which will eventually be depleted if alternative energy sources are not found. Further, the burning of fossil fuels to produce electricity emits greenhouse gases that contribute to global warming. Nuclear power presents environmental and nuclear proliferation hazards.
Solar energy and wind power are promising alternative energy sources that can reduce reliance on fossil fuels for generating electricity. Solar energy and wind power are renewable resources so there is no concern about future depletion of these resources. Further, the generation of electricity from solar energy and wind power does not emit greenhouse gases and is therefore considered more environmentally friendly. Also, generation of electricity from renewable energy sources does not generate hazardous by-products that need to be disposed of.
In the field of solar energy utilization, much work has been done to develop a system that is economical enough to replace the combustion of fossil fuels to supply the growing needs of our ever more highly populated planet in a clean and renewable way. Solar photovoltaic systems have received the most attention over the years and yet suffer from a very high cost of equipment. Even with government subsidies, 25 year payback periods are common. In addition, because electricity is very expensive to store, the use of periodic energy sources such as solar to produce electric power is problematic.
A better approach would be to use solar energy to produce a fuel, which could be stored and transported easily and economically. This fact has led to a great deal of research into ways to use sunlight to produce fuels from low energy feedstock, most notably to produce hydrogen from water. This arises largely from the well-known fact that hydrogen, when used as a fuel produces only water, which truly makes hydrogen a “clean fuel” candidate. Despite the large body of work on this topic there is not yet an economical process in practice.
Electricity for electrolysis may come from renewable resources such as solar or wind, but ˜60 kWhr is required to produce 1 kg of hydrogen from water electrolytically. This limitation renders this process too expensive to compete in the energy marketplace with fossil fuels.
Direct thermal decomposition of water has been proposed as a possible way to avoid the inefficiencies and expense of the photon-to-electron conversion step that limits the aforementioned solar electrolysis system approach. The follow formula illustrates the decomposition:2H2O+Heat→2H2+O2 To generate thermal decomposition of water, very high temperatures are required to produce appreciable amounts of reaction products, i.e., hydrogen and oxygen. This imposes very strict requirements on reactor materials, because they are exposed to very high temperatures and very reactive gasses. Thermal shock and very large thermal gradients are also a concern because of mismatches in thermal expansion coefficients for different construction materials. Besides thermal and chemical stability, the direct thermal decomposition system should also be able to allow for the separation of the reaction products, oxygen and hydrogen from each other to avoid recombination. In order to accomplish this separation, the gases must either be cooled rapidly and then separated later or separated when hot. Rapid cooling or quenching while potentially effective at preventing recombination, also limits the process in 2 important ways. First, it limits how far the decomposition reaction may proceed to what is formed at equilibrium. Second, it inevitably results in significant heat loss during quenching as all of the unreacted water must be cooled along with the product gases.
Separating the hydrogen and oxygen gases while hot may be accomplished by using ceramic-based high temperature hydrogen permeable membranes and/or high temperature oxygen permeable membranes. As a result, work in this area has taken place, however successful implementation of a commercially successful system with sufficient robustness and gas throughput has not occurred.
In general, membranes large enough to have sufficient gas throughput also have to be very thick to maintain structural integrity, which reduces gas permeability. Another limitation to gas throughput is available surface area. Gas permeation rates are generally linearly proportional to membrane surface area, and, for very high temperature reactors, the available surface area can be quite limited. For example, a 5 square meter (m2) solar collector is capable of supplying about 5 kW of heating power from the sun to a high temperature reactor. In order to achieve the temperatures needed to cause thermal decomposition of water, a concentration factor of about 5,000 times might be needed. This means that the 50,000 cm2 of sunlight collection area would need to be focused down to approximately a 3.5 cm diameter spot. Assuming 20% of this energy is used to decompose water, approximately 2 liters per minute of oxygen will need to be removed from the system during operation so that the hydrogen produced could be preserved. Any high-temperature reactor system that uses one or more of the decomposition chamber walls as the separation medium, as has been proposed in the past, would not be able to achieve the required surface area (at least 2000 cm2) in a practical manner. In addition any attempt to supply the require membrane area in such a scheme would then also require a thickness to sustain the pressure differentials required for oxygen separation, which would further reduce oxygen permeation capability. Altogether, such a system has a gas permeation insufficiency of 100 to 1000×.